It is known in the arts of electrophotography and electrostatic printing to tone electrostatic latent images contained on recording member surfaces by the application thereto of electroscopic marking particles. Such electroscopic marking particles may conveniently be applied to recording member surfaces by means of magnetic applicators of various types. In the oldest so-called magnetic brush method of magnetic application of electroscopic marking particles such marking particles are admixed with iron powder or the like and carried on a magnet roller into contact with the latent image bearing recording member surface. The magnet roller used in the magnetic brush applicator normally comprises a central core containing longitudinally two active magnetic poles in a fixed and non-moving configuration, contained within a rotating non-magnetic sleeve. One magnet pole of the so-called pick-up magnet is located at an appropriate position within the sleeve to attract magnetic material thereto which is transported by the rotating sleeve to the toning area, at which area the second magnet pole causes the magnetised particles of toner carrier in conformity with the lines of force to form the so-called brush comprising chains which are substantially normal to the rotating sleeve surface. The toner particles are attracted from the brush by the electrostatic latent image on the recording member surface to deposit on such surface forming a visible image deposit thereon.
In more recently developed technology the magnetisable carrier particles are not used, and the toner itself contains sufficient magnetic material to be attracted to a magnetic roller and carried thereby. Such so-called single component magnetic toners are normally applied by means of magnet rollers containing a multiplicity of magnet poles, ranging from about 6 poles as disclosed in U.S. Pat. No. 4,142,165 of Miyakawa et al. to 12 poles as illustrated in U.S. Pat. No. 4,081,571 of Nishihama et al. Magnetic pole arrangement may be alternate north south or may contain two adjacent poles of like polarity as disclosed for instance in U.S. Pat. No. 4,122,456 of Berkowitz et al. Other multipole magnet rollers are disclosed for instance in U.S. Pat. Nos. 4,165,393 of Suzuki et al., 4,162,842 of Wu, 4,154,520 of Nishikawa, 4,142,281 of Muller, 4,121,931 of Nelson, 4,063,533 of White, 4,003,334 of Samuels et al., 3,909,258 of Kotz and 3,882,821 of Katayama et al.
Such multiple magnet rollers may be of the rotating core or rotating sleeve type, or each of the core and the sleeve may rotate at different speeds. Alternatively the core and sleeve may be fixed to each other and rotate as a unit. Most modern practice appears to be directed towards the use of a rotating sleeve with a fixed magnet core, with one of the poles aligned with the actual toning area to cause the toner particles to form chains which are normal to the sleeve surface at this position.
Prior art investigators have generally considered that it is advantageous to use multipole magnets in magnetic rollers because in such case the toner moves with a tumbling action from one pole to the next, and this is considered to be advantageous with regards toner distribution on the magnet roller at the toning position.
A disadvantage of the prior art multipole magnet rollers is the permanence of the magnetic field, which causes the roller sleeve to remain more or less covered with toner at all time. This can cause some degree of magnetic saturation of the toner particles contained on the magnet roller and in addition makes it difficult to clear the toner entirely from the magnet roller at the toning position.
The present invention overcomes these prior art disadvantages.